Before any well drilling is initiated, engineers create a well plan and architectural diagram, showing the hole and casing sizes needed to drill the well to its desired depth. A typical wellbore architectural diagram for an onshore well is shown in FIG. 1. Normal drilling operations include drilling the hole and adding new joints of pipe, one at a time, as the hole deepens. It also involves “tripping” the drill string all the way out of the hole to put on a new bit and then running it back to the bottom (making a round trip). Other key steps include running and cementing the large-diameter steel casing used to seal selected intervals of the hole.
It is standard practice to drill a well bore of large diameter to a certain depth and then line the drilled well bore with a casing, which is cemented in place. This is sometimes called surface casing. Thereafter, smaller size bits and casing are used, and the borehole is continued, again cementing the intermediate casing in place. This step can be repeated several times with decreasing diameter pipe, depending on the depth of the well. Thus, a borehole is comprised of a series of borehole sections of decreasing diameter, each having a string of pipe cemented in place. Eventually, the hydrocarbon-containing zone is reached, and that section of well typically contains slotted liner, the small slots or holes allowing the entry of oil.
In the case of the single stage cementing operation, the casing with all of the required cementing accessories such as the float collar, centralisers, and the like is run into the hole until the shoe is just a few feet off the bottom of the hole and the casing head is connected to the top of the casing. It is essential that the cement plugs are correctly placed in the cement head. The casing is then circulated to clean it before the cementing operation begins. At least one casing volume should be circulated to clean the well and annular space of drilling mud, which can contaminate and compromise the cement. The first cement plug (wiper plug), is pumped down ahead of the cement to wipe the inside of the casing clean. The spacer is then pumped into the casing. The spacer is followed by the cement slurry, and this is followed by the second plug (shut-off plug). FIG. 2 shows a variety of bottom and top wiper plug styles.
When a bottom wiper plug with rupturable diaphragm, as shown in FIG. 3, reaches the float collar, its rubber diaphragm is ruptured, allowing the cement slurry to flow through the plug, around the shoe, and up into the annulus. At this stage, the spacer provides a barrier to mixing of the cement and mud. When the solid, shut-off plug reaches the float collar it lands on the wiper plug and stops the displacement process. The pumping rate should be slowed down as the shut-off plug approaches the float collar and the shut-off plug should be gently bumped into the bottom, wiper plug. The casing is often pressure tested at this point in the operation.
The pressure is then bled off slowly to ensure that the float valves, in the float collar and/or casing shoe, are holding. The displacement of the top plug is closely monitored, and the volume of displacing fluid necessary to bump the plug is calculated before the job begins. When the pre-determined volume has almost been completely pumped, the pumps are slowed down to avoid excessive pressure when the plug is bumped. If the top plug does not bump at the calculated volume (allowing for compression of the mud) this may be because the top, shut-off plug has not been released. If this is the case, no more fluid should be pumped, since this would displace the cement around the casing shoe and up the annulus. Throughout the cement job, the mud returns from the annulus should be monitored to ensure that the formation has not been broken down. If formation breakdown does occur, then mud returns would slow down or stop during the displacement operation.
One problem with prior art wiper plugs is fin degradation. Because the wiper plug material is in contact with the inner diameter of the tubular, it experiences wear that can cause the fins to degrade and eventually fail. Failed fins will not wipe the pipe clean, leaving fluids and/or debris behind, causing problems for future well operations. Debris left inside the casing can plug up tools or perforations during completion operations, cause wear on tools, and impede flow.
Previous attempts to correct this problem have simply used more fins in the hope that they will not wear out before the operation is completed. However, the fins are still subject to the same wearing forces, and still tend to degrade at roughly the same rate, even if more are present. Another solution is to use more durable materials for the fins. However, the fins still need to be flexible to achieve their intended purposes and to traverse through less than perfect tubing, and thus there is a limit on how durable the fin material can be. Further, more durable materials still degrade, and this solution only delays the onset of the problem.
Thus, what is needed in the art are better wiper plugs that can provide additional and better wiping, especially closer to the tail end of the well bore where prior art fins have already degraded by the time they reach the tail end.